1. Field of the Invention
This invention relates to a brushless synchronous motor comprising a short-circuited protective winding associated with the field winding of the rotor.
2. Description of the Prior Art
It is desirable to limit the voltages which are induced in the field winding of the rotor of a synchronous motor when the latter is started from standstill and to protect the switching means which are required for the feeding of the field winding and consist particularly of rectifiers and transistors from excessive overvoltages during an asynchronous start of the motor. This is usually accomplished in that the field winding is shunted by a protective resistor. By the provision of a protective resistor which has a resistance of an order of five to ten times the rotor resistance and shunts the field winding of the rotor, the voltage that is induced in the field winding is limited to a permissible value and the residual slip occurring during the transition from the asynchronous start to the synchronized operation is decreased. Besides, the hunting torque which occurs near the synchronous speed can be highly reduced so that the starting characteristics are generally improved. But the provision of such protective resistors involves the disadvantage that the resistors must be accommodated in the rotor so that expensive mounting means are required to take up the centrifugal force. Besides, the protective resistors result in a Joule's heat loss even during normal operation because the protective resistors always hunts the field winding of the rotor.
In a known circuit designed to avoid said disadvantages the protective resistor has been replaced by a short-circuited protective winding, in which currents are induced during the asynchronous start and attenuate the alternating fields established adjacent to the field winding. During a synchronous operation the rotating field does not move relative to the rotor and to the protective winding so that losses which would adversely affect the efficiency of the motor cannot occur during normal operation. Another advantage resides in that a stable structure is obtained because additional mechanical supports are not required. But by a short-circuited protective winding of that kind the asynchronous torques and the hunting torques can be influenced only because the protective winding is interlinked with the stator field so that the design of a synchronous motor for desired starting characteristics is adversely affected.